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United States Patent |
5,091,066
|
Foa
,   et al.
|
February 25, 1992
|
Electrochemical synthesis of 2-methyl-5-pyrazinoic acid
Abstract
There is disclosed a process for the preparation of 2-methyl-5-pyrazinoic
acid consisting of electrochemically oxidizing a compound having formula:
##STR1##
wherein X is --OH, Cl, Br, --O--CO--R, --O--SO.sub.2 --R, in which R is a
C.sub.1 -C.sub.5 alkyl radical, optionally substituted with F or Cl, or it
is a C.sub.6 -C.sub.12 aryl radical, in an aqueous alkaline medium, at a
temperature ranging from 20.degree. C. to 90.degree. C., using anodes
coated with nickel oxide-hydroxide.
Inventors:
|
Foa; Marco (Novara, IT);
Forlini; Fabrizio (Pavia, IT);
Gatti; Norberto (Novara, IT);
Borsotti; Giampiero (Novara, IT)
|
Assignee:
|
Presidenza del Consiglio dei Ministri (Rome, IT)
|
Appl. No.:
|
707310 |
Filed:
|
May 29, 1991 |
Foreign Application Priority Data
| Nov 19, 1987[IT] | 22693 A/87 |
Current U.S. Class: |
205/426 |
Intern'l Class: |
C25B 003/02 |
Field of Search: |
204/72,78
544/336
|
References Cited
U.S. Patent Documents
4488944 | Dec., 1984 | Stutts et al. | 204/79.
|
4496440 | Jan., 1985 | Campbell et al. | 204/78.
|
4640750 | Feb., 1987 | Blickle | 204/79.
|
Foreign Patent Documents |
2099820 | Dec., 1982 | DE.
| |
Other References
Kaulen et al., "Oxidation of Primary Alcohols to Carboxylic Acids at Nickel
Hydroxide Electrode", Synthesis, pp. 513-516, Jul. 1979.
Barlin, G. B., "The Pyrazines", John Wiley & Sons, New York, 1982.
|
Primary Examiner: Niebling; John
Assistant Examiner: Marquis; Steven P.
Attorney, Agent or Firm: Morgan & Finnegan
Parent Case Text
This is a continuation of co-pending application Ser. No. 07/271,675, filed
on Nov. 16, 1988.
Claims
What we claim is:
1. A process for the preparation of 2-methyl-5-pyrazinoic acid consisting
in subjecting to electrochemical oxidation a compound having formula:
##STR7##
wherein X is Cl, Br, --O--CO--R, --O--SO.sub.2 --R, in which R is a
C.sub.1 -C.sub.5 alkyl radical, optionally substituted with one or more
atoms of F or Cl, or it is a C.sub.6 -C.sub.12 aryl radical, in an
electrochemical cell, by using anodes coated with NiO(OH) nickel
oxide-hydroxide, in an aqueous medium containing at least 6 equivalent
moles of a base per mole of compound (I), at a temperature ranging from
20.degree. to 90.degree. C.
2. A process according to claim 1, wherein the aqueous alkaline medium
further contains an organic solvent for compound (I), which solvent is
miscible with water and inert under the reaction conditions.
3. A process according to claim 2, wherein the solvent is acetonitrile.
4. A process according to claim 1, wherein the concentration of compound
(I) in the aqueous alkaline solution ranges from 0.01 and 1 mole/liter.
5. A process, according to claim 1, wherein the base is selected from the
group comprising hydroxides, carbonates or bicarbonates of alkaline or
alkaline earth metals.
6. A process according to claim 1, wherein in the electrochemical oxidation
use is made of current density ranging from 5 to 100 mA/cm.sup.2.
Description
FIELD OF THE INVENTION
The present invention relates to a process for the preparation of
2-methyl-5-pyrazinoic acid by electrochemical oxidation of a functional
derivative of 2,5-dimethylpyrazine, on a nickel oxide-hydroxide anode.
BACKGROUND OF THE INVENTION
2-methyl-5-pyrazinoic acid is the key-intermediate for the preparation of
ACIPIMOX.RTM. 2-methyl-5-pyrazinoic-1-oxide, an important antihypertensive
drug.
From U.K. patent 2,099,820 it is known, how to prepare
2-methyl-5-pyrazinoic acid by condensation of pyruvic aldehyde with
diaminodicyanoethylene and subsequent reaction of thus obtained
5-methyl-2,3-dicyanopyrazine with an acid in an aqueous medium.
The reaction between 5-methyl-2,3-dicyanopyrazine with the acid, however,
is not selective and gives rise to the formation of equimolecular mixtures
of 2-methyl-5-pyrazinoic acid and of 2-methyl-6-pyrazinoic acid,
exhibiting the further drawback connected with the separation of said
acids.
Moreover from G. B. Barlin "THE PYRAZINES" JOHN WILEY. Ed. page 79 (1982),
it is known, how to prepare 2-methyl-5-pyrazinoic acid by oxidation of
2,5-dimethylpyrazine or of 2-methyl-5-oxymethylpyrazine with potassium
permanganate.
The above described process, however, proves not to be suitable for an
industrial economic production, mostly owing to the considerable amounts
of potassium permanganate, which need to be used (from 1 to 2 moles of
KMnO.sub.4 per mole of substrate) and to the problems connected with the
separation and disposal of large amounts of wastes.
Moreover, the reaction between 2,5-dimethylpyrazine with KMnO.sub.4 is not
selective and gives rise to the formation of considerable amounts of
2,5-dicarboxypyrazine as well.
From J. Kaulen et al.--"Synthesis", 513-516 (1979) it is also known, how to
electrochemically oxidize primary alcohols on NiO(OH) nickel
oxide-hydroxide anode.
DETAILED DESCRIPTION OF THE INVENTION
It was now found that 2-methyl-5-pyrazinoic acid can be obtained with high
yields and conversions by electrochemical oxidation of
2,5-dimethylpyrazine derivatives on anodes coated with NiO(OH) nickel
oxide-hydroxide.
Therefore the object of the present invention is a process for the
preparation of 2-methyl-5-pyrazinoic acid consisting in subjecting to
electrochemical oxidation a compound having general formula:
##STR2##
wherein: X represents OH, Cl, Br,
##STR3##
in which R is a C.sub.1 -C.sub.5 alkyl radical, optionally substituted
with one or more atoms of F or Cl, or it is a C.sub.6 -C.sub.12 aryl
radical, in an electrochemical cell, by using anodes coated with NiO(OH)
nickel oxide-hydroxide, in an aqueous alkaline medium, containing at least
5 equivalent moles of a base per mole of compound (I), when X.dbd.OH or at
least 6 equivalent moles of a base per mole of compound (I), when X is Cl,
Br,
##STR4##
and, optionally an organic solvent for compound (I), which solvent is
miscible with water and inert under the reaction conditions, at a
temperature ranging from 20.degree. to 90.degree. C.
The anodic reaction can be schematized as follows:
##STR5##
The concentration of compound (I) in the aqueous basic solution is not
critical and can vary between 0.01 and 1 moles/liter, preferably between
0.03 and 0.8 moles/liter.
Hydroxides, carbonates, bicarbonates of alkaline and alkaline-earth metals
can be used, for instance, as base.
Tertiary butyl alcohol, tertiary amyl alcohol, acetonitrile can be used,
for instance, as organic solvents, which are mixable with water and inert
under the reaction conditions.
Such solvents are used preferably to obtain the best yields in
2-methyl-5-pyrazinoic acid (II), when in compound (I), X is Cl or Br.
In the oxidation reaction use can be made of current densities ranging from
5 to 100 mA/cm.sup.2, preferably from 8 to 70 mA/cm.sup.2.
The quantity of current necessary for the complete conversion of compound
(I) into compound (II) can range from 4 to 10 F/mole.
The temperature of oxidation reaction generally ranges from 20.degree. to
90.degree. C., preferably from 30.degree. to 70.degree. C.
When the electrochemical oxidation reaction is over, the reaction mixture
is acidified up to isoelectric pH of 2-methyl-5-pyrazinoic acid (about pH
1.5).
2-Methyl-5-pyrazinoic acid can be extracted, using an organic solvent, from
the resultant solution, after having salted the solution or after having
evaporated the water.
Compounds of formula (I) are known or they can be prepared by known
methods, from 2,5-dimethylpyrazine or from its derivatives.
For instance, the compounds of formula (I), when X is Cl or Br, can be
prepared by reaction of 2,5-dimethylpyrazine with the customary
halogenation agents such as N-chloro-(bromo)-succinimmide or with sulfuryl
chloride.
The compounds of formula (I), when X is
##STR6##
can be obtained by reaction of N-oxide of 2,5-dimethylpyrazine with acetic
anhydride, or by exchange reaction between
2-halogenmethyl-5-methylpyrazine with an alkaline acetate.
The compounds of formula (I), when X is --OH, can be obtained by hydrolysis
of the corresponding halogen derivative or of the corresponding
acyloxymethyl or organic sulfonates.
The compounds of formula (I), when X is --O--SO.sub.2 --R can be obtained
from the corresponding 2-halogenmethyl-5-methylpyrazines by exchange with
an alkaline sulfonate.
The cathodic material of the electrochemical cell is not critical, and it
generally consists of stainless steels of different kind.
The NiO(OH) nickel oxide-hydroxide anode is prepared, as known from the
prior art, by electrolysis, in an electrochemical cell, of an aqueous
alkaline solution of a nickel salt, by changing the electrode polarity by
cycles.
The time required for every cycle can range within large limits, from very
few seconds to some minutes, taking care that the electrode, which will be
used as anode in the reaction of electrochemical oxidation, remains in
anodic polarity for a longer time compared with the counter-electrode.
The cycle number may range within large limits as well, according to the
desired thickness of NiO(OH) on the electrode and to the modalities. The
cyclic polarization of the electrodes is carried out.
The preferred materials going to make up the electrode, which will be used
as work electrode in the oxidation reaction, are stainless steels, nickel,
noble metals belonging to the platinum group, in the form of a sheet or a
net.
The material, the counter-electrode consists of, is not critical; and it
generally consists of stainless steels of different kind.
The preferred nickel salt is sulfate, other nickel salts may be, however,
used, such as, for instance, nitrate.
The salt concentration generally ranges from 0.05 to 2 moles/liter,
preferably from 0.08 to 1.5 moles/liter.
The aqueous solution, wherein the nickel salt is dissolved, contains sodium
acetate in an amount, that is substantially equimolecular with respect to
the nickel salt and hydroxides of alkaline metals ranging from 0.003 to
0.01 moles/liter, preferably from 0.004 to 0.006 moles/liter.
For the preparation of the electrode coated with NiO(OH) use is made of
densities of current ranging from 1 to 25 mA/cm.sup.2, preferably from 3
to 10 mA/cm.sup.2.
A few examples will be given, by way of illustration, but not of
limitation.
EXAMPLE 1
Preparation of 2-methyl-5-pyrazinoic acid by oxidation of
2-methyl-5-oxymethylpyrazine
1) Preparation of the electrode coated with NiO(OH)
99 ml of H.sub.2 O, 2.93 g (10.3 millimoles) of NiSO.sub.4.7H.sub.2 O, 0.96
g (11.6 millimoles) of sodium acetate, 1 ml of a solution of 0.5N sodium
hydroxide (0.0005 moles) were introduced into an undivided electrochemical
cell containing an anode consisting of a nickel net being 24 cm.sup.2 in
surface and a cathode consisting of a Incoloy 825 wire. The nickel net
electrode was polarized by anodic way for 60 seconds by passing a constant
current of 120 mA, afterwards the polarity was inverted for 10 seconds.
The cycle was repeated 10 times and at the end the net electrode was kept
for 2 minutes under anodic polarity. At the end of this procedure, the
nickel net electrode, a NiO(OH) layer had deposited on, was rinsed with
water and used in another electrochemical cell for the subsequent
oxidation reaction.
2) Electrochemical oxidation
90 ml of an aqueous solution containing 3.6 g of NaOH and 0.480 g (3.87
millimoles) of 2-methyl-5-oxymethylpyrazine were introduced into an
undivided electrochemical cell which contained the anode coated with
NiO(OH), prepared as above described and a cathode which consisted of an
Incoloy 825 wire.
The temperature was brought to 40.degree. C. and the electric current was
passed at a constant density of 12 mA/cm.sup.2. The electrolysis was
continued till the passage of 6 F per mole of
2-methyl-5-oxymethylpyrazine.
The reaction crude product was acidified with HCl up to a pH of about 1.5,
the water was evaporated and the residue was extracted by means of
methylethylketone. From the extract, after evaporation of the solvent, 3.6
millimoles of 2-methyl-5-pyrazinoic acid were recovered, with a yield of
93%.
EXAMPLE 2
100 ml of an aqueous solution containing 6 g of NaOH and 2.56 g of
2-methyl-5-acetoxypyrazine were introduced into an undivided
electrochemical cell containing an anode coated with NiO(OH), prepared as
described in example 1 and a cathode consisting of an Incoloy 825 wire.
The reaction mixture was electrolyzed at 40.degree. C., with a constant
density of current, of 12 mA/cm.sup.2 till the passage of 9.000 Coulomb.
When the electrolysis was over, the reaction crude product was acidified
with HCl up to a pH of about 1.5, the water was evaporated and the residue
was extracted by means of methylethylketone.
From the extract, after evaporation of the solvent, 1.893 g of
2-methyl-5-pyrazinoic acid were recovered, with a yield of 89%.
EXAMPLE 3
A solution consisting of 15 g of K.sub.2 CO.sub.3 and 10 g of tertiary
butyl alcohol dissolved in 70 ml of water and 0.684 g of
2-chloromethyl-5-methylpyrazine were introduced into an undivided
electrochemical cell containing an anode coated with NiO(OH), prepared as
described in example 1, and a cathode consisting of an Incoloy 825 wire.
The reaction mixture was electrolyzed at 60.degree. C., with a constant
density of current of 12 mA/cm.sup.2, till the passage of 6 F per mole of
2-chloromethyl-5-methylpyrazine.
When the electrolysis was over, the azeotropic mixture H.sub.2 O/terbutanol
was evaporated from the reaction crude product; one diluted slightly with
H.sub.2 O, one acidified with dilute HCl up to a pH of about 1.5, one
dried, afterwards the residue was extracted by means of methylethylketone.
From the extract, after evaporation of the solvent, 0.491 g of
2-methyl-5-pyrazinoic acid was recovered, with a yield of 74%.
Although the invention has been described in conjunction with specific
embodiments, it is evident that many alternatives and variations will be
apparent to those skilled in the art in light of the foregoing
description. Accordingly, the invention is intended to embrace all of the
alternatives and variations that fall within the spirit and scope of the
appended claims.
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